This invention relates generally, to the recovery of natural gas from shale and more particularly, to an improved method and apparatus for such recovery in situ.
The vast extent of organic sedimentary deposits underlying the U.S. land area has long been recognized for its potential yield of energy-rich fuels. In terms of land mass, the largest deposit comprises the Devonian-Mississippian black shale composite while the highest organic content is found in the Green River formation, located in Colorado, Wyoming and Utah.
Following the recongized petroleum crisis of the past decade, billions of dollars were expended by industry and governments to research and develop methods and apparatus for recovering oil, and to some extent gas, from this shale. For the most part these efforts have been shelved and no known commercial production of oil or gas from shale is evident in this country. This is attributable to several factors, not the least of which is economics. In the case of above ground processes, shale is mined and then retorted to extract the oil and/or gas therefrom, following which the spent shale must be disposed of. The capital expenditures of such an operation are enormous even when terrain, accessibility and availability of disposal areas are of minimal concern. The alternative recovery process involves in situ operations wherein bore holes drilled into a subterranean shale deposit are combined with various apparatus intended to recover oil and/or gas from the surrounding shale.
Although the above latter approach substantially reduces the handling and disposal problems attendant with the above-ground mining process and thus curtails the overall capital outlay, the efficiency and productivity of existing processes fall far short of that required for an economical operation. In the case of oil recovery, the very impermeability of oil shale beds requires the employment of means, not heretofore adequately developed, to fracture or otherwise make available the shale deposit for whatever retort process is being utilized for the extraction of the oil therefrom. In the case of gas recovery, to which this invention is especially directed, no known process or apparatus, until the instant development, has yet demonstrated an economical manner for taking advantage of this impermeability factor to yield natural gas from the shale in situ, particularly in view of the drop in world oil prices from the levels of the past decade.
To be considered economically feasible, a recovery system must be capable of functioning when applied to shale as located at any depth, even at the most minimum of depths such as when the overburden may extend only five feet in depth, thereby avoiding the necessity of drilling bore holes of extreme depths. Additionally, a viable system should not require any moving parts and should, once in place and operational, be capable of producing commercially acceptable gas for an extended period of time, such as for five years. And even more important, the gas as produced should continuously yield over 70 MCF per day at no less than 800 BTU. To accomplish all of the foregoing, improved heating means associated with appropriate control means must be provided and operated within strict parameters, as proposed by the present invention.